1. Field of the Invention
This invention relates to conversion of film to video, post-production of video, conversion of video to film, and conversion between different video formats.
2. Description of the Prior Art
A vast amount of film masters are in a 24 frame/s or 25 frame/s format, and often it is required to convert the film to video for distribution, post-production and broadcast transmission, and to convert the video back to film as a sub-master. One standard format for video employs interlaced fields at 60 field/s. For example, the SMPTE 240M high definition format employs 1125 line/frame, 2:1 interlace, 60 field/s. Because of the difference in frame rate between the film and video, there is no one-to-one correspondence between the film frames and the video frames, and this presents a problem.
A partial solution to this problem in the case of 24 frame/s film (and also 25 frame/s film if a slightly incorrect speed is not unacceptable) is to use a 3232 format for the video. With 3232, there is a correspondence between a series of four film frames and ten video fields as follows:
Film Video Frame 1 Frame 1 odd field Frame 1 Frame 1 even field Frame 1 Frame 2 odd field (additional) Frame 2 Frame 2 even field Frame 2 Frame 3 odd field Frame 3 Frame 3 even field Frame 3 Frame 4 odd field Frame 3 Frame 4 even field (additional) Frame 4 Frame 5 odd field Frame 4 Frame 5 even field
It will therefore be appreciated that the real-time relationship between 24 frame/s film and 60 field/s video is maintained (at least when averaged over a four film frame period), but that two of the video fields, frame 2 odd and frame 4 even, are duplicated, and so the fields of video frame 2 do not correspond to a single film frame and likewise with the fields of video frame 3. During post-production of 3232 video, for example combining two 3232 video signals, there is the problem of synchronising the additional fields of the signals, so that if the video is later converted back to film the additional fields can be dropped and the other fields can all be maintained. Subsequent conversion to 50 field/s video compounds the problem, because every sixth field needs to be dropped. Sometimes the dropped field will be an additional field, and sometimes it will not.